1. Field of the Invention
The present invention relates to a navigation system for a vehicle which guides a vehicle driver along a route to a destination and, more particularly, to a navigation system for a vehicle which is capable of finding an optimum route.
2. Description of the Related Art
The navigation system for a vehicle has various functions for guiding a vehicle driver along a route specified by a driver. One of the functions is to re-search for a route when the vehicle goes off a specified route and to restart the guiding along the found route.
A driver may sometimes go off a specified route when driving according to instructions about the route. As a countermeasure for this event, for example, Japanese patent application laid-open No. HEI1-173815 has disclosed such a function for immediately informing a driver that the vehicle has been off a set route and re-searching for a new route. However, the system of Japanese patent application laid-open No. HEI1-173815 has a problem in that it takes a long time searching for a new route because the disclosed system searches over all routes from its current position to a destination when a vehicle goes off the route. Japanese patent application laid-open No. HEI6-68389 discloses a navigation system for a vehicle which is intended to solve the above-described problem.
According to the aforementioned Japanese patent application laid-open No. HEI6-68389, when the system determines that a vehicle is off a specified route, a re-search switch is displayed, and when operation of the re-search switch is detected, the system executes a search of a route for returning to the previous route. In this system, when an operation of searching or re-searching for a route is detected, a route is searched for with preference for the traveling direction of a vehicle. Because a route is found with preference for the traveling direction of a vehicle as described above, if there is only one unforked road going through, for example, a mountainous area, the system may sometimes guide along a roundabout route. Thus, there is a problem in that the system may guide along a route which is not favorable for a user.